Because she wants to expeirience and learn society where it happens.
Internet is main space for human interaction. Asking why 11 year old would be on the internet is like asking why would she be in the mall or on the street or anywhere really except home, school and inside of family car.
She should be on the internet and she should be expeiriencing it because that's the only way she can learn to tolerate it and find her way around it. She won't learn it at school or from parents that barely have any clue what's internet outside of their email accounts.
When children access internet you need to keep an eye and explain parts of it you yourself do understand. Same as with TV or people in the streets. You can't ban things till children become adults because you are stealing from them opportunity to learn when learning is easiest for them.
If they have your support internet won't break them.